1. Field
The present disclosure relates to a nickel electrolcatalyst coated with an ultralow loading amount of platinum, a method for preparing the same and an anion exchange membrane water electrolyzer using the same.
[Description about National Support Research and Development]
This study is made by the support of Core Research Business of Korea Ministry of Science, ICT and Future Planning under the supervision of Korea Institute of Science and Technology and the research subject title is ‘Development of Advanced Low-Temperature CO2 Electrolysis Technology for Production of Synthesis Gas’ (Subject Identification No.: 1711018888); and of Korea Ministry of Trade, Industry and Energy under the supervision of Korea Institute of Science and Technology and the research subject title is ‘Development of Non-Platinum Catalysts Technology for Lowing Price of Fuel Cell for Automobile’ (Subject Identification No.: 20133010011320).
2. Description of the Related Art
It has been recognized that water electrolysis is a technology for producing hydrogen through an eco-friendly process related with a renewable electric energy source, such as solar heat and wind force. As electrolyte for a water electrolysis cell, an alkaline liquid electrolyte used in combination with a diaphragm separator has been developed and used widely. However, recently, a solid polymer electrolyte water electrolyzer (referred to as SPEWE hereinafter) has been given many attentions by virtue of its various advantages and has accomplished high efficiency, hydrogen production rate and hydrogen purity while allowing operation under high voltage. For SPEWE, two types of solid polymer electrolytes have been studied: a proton exchange membrane and anion exchange membrane. Since a solid polymer electrolyte and ionomer (e.g. Nafion) provide a higher hydrogen production rate as compared to an anion exchange membrane water electrolyzer (referred to as ‘AEMWE” hereinafter), a proton exchange membrane water electrolyzer (referred to as ‘PEMWE’ hereinafter) has been regarded as highly effective candidate for a process for producing hydrogen. However, the extremely acidic condition of PEMWE requires the use of a noble metal both at a cathode and an anode. Thus, the high cost of PEMWE hinders the commercialization thereof.
In general, platinum-group-metals, including Pt, Pd and Rh, are used as electrocatalyst for hydrogen evolution at a cathode. Noble metal oxides (e.g., RuO2 and IrO2) and a mixture thereof are used currently for oxygen evolution at an anode by virtue of their high activity and stability. In addition, the severe oxidization environment of PEMWE requires not only a noble metal catalyst but also an expensive Ti-based current collector and separator.
Meanwhile, non-noble metal catalysts may be used both for hydrogen evolution and for oxygen evolution in AEMWE. To provide a membrane electrode assembly (referred to as ‘MEA’ hereinafter), porous electrodes have been used generally and catalyst powder have been coated onto a membrane or gas diffusion layer in combination with a binder. Typically, such a type of electrode has a metal loading amount of several milligrams per unit area and a thickness of several micrometers.
Using a large amount of non-noble metal catalyst leads to improvement of the quality of a cell. However, such a thick catalyst layer causes an increase in resistance against mass transfer of reactants/products and degradation of catalyst availability. Particularly, a drop in ohmic voltage at a gas emission electrode becomes significant under high current density due to the hydrogen and oxygen bubbles locked in the pore of a catalyst layer or gas diffusion layer. Another disadvantage of a thick catalyst layer having a high catalyst loading amount is low catalyst availability caused by low ion conductivity of a hydroxide conductive ionomer. In addition, even though non-noble metals are cheaper than Pt-group-metals, such a high catalyst loading amount causes an increase in cost undesirably.
Meanwhile, instead of conventional porous electrodes, particle-type electrodes obtained by direct formation of a catalyst on a Pt-group metal surface have been expected to facilitate mass transfer and to provide high catalyst availability. However, a low loading amount results in deficiency of active sites, thereby limiting the quality of a cell in actual application.